In nuclear reactors of safety type, constructed in accordance with the Secure concept with an upper and a lower density lock, the density locks constitute an important safety function. With a density lock, two liquid systems can be separated from each other at a boundary by a warmer liquid system being superimposed on a colder liquid system. During normal operating conditions, the density locks prevent the warmer water from a primary cooling system from being mixed with the colder water from a secondary cooling system (pool water). At an increased temperature level, the density locks permit water to flow through, among other things for mixing the warmer water with the colder water to reduce the increased temperature level.